Understanding Fixed vs. Interchangeable Probe Humidity Transmitters

Differences and Benefits of Transmitters with Interchangeable Probes vs. Fixed Instrumentation

In this blog we will outline some basic concepts around transmitters identifying various technical differences that you should be aware of when considering instrumentation for your application. An important area we will focus on is the differences and benefits of transmitters with interchangeable probes vs fixed probes.

Firstly, what is a transmitter?

For many the term transmitter is confusing. The name "transmitter" derives from the instrument’s primary function: to transmit data. This is a general term used across various industries and applications, from radio broadcasting to process control in industrial settings.

In the case of measurement transmitters, the device captures data using a probe or sensor, converts this data into an interpretable signal, and then transmits this signal. This allows for centralized monitoring, data logging and controlling other devices based on the transmitted information.

Modern instruments can transmit data via wires (wired transmitters) or using radiofrequencies (wireless). In this blog we are only discussing wired transmitters.

Wired Transmitters: Modern instruments can transmit data via wires. Understanding the kind of signals they use is essential:

Analog Signals: These are continuous signals. Examples include:

• 4…20mA: Commonly used for its resistance to interference.
• 0…10v: Offers more precise measurements but can be affected by electrical noise and voltage drop over long distances.
• Logic Relays (On/Off): Simplest form, used for binary actions.

Digital Signals: They transmit discrete signals, additional data. Digital signals are often more immune to noise and can be more secure through encryption. Examples are:

• Modbus: Suitable for connecting multiple devices on the same network.
• Profi-Bus: Designed for communication in automation technology.
• HART: Combines analog and digital data, providing enriched information.
• HTTP: More contemporary, useful for IoT devices.

Many different digital and analog signals are available in instrumentation, it is important to understand what signals you require, typically this is defined by the systems you are connecting your devices to. Different signal outputs can affect cost and quality of measurements.

Fixed vs. Interchangeable RH Probe Transmitters

One of the key choices to understand when selecting the right humidity transmitter is whether it has a fixed vs interchangeable probe. The probe is the measurement part of the transmitter.

Fixed RH Probe Transmitters

• Integrated Design: In fixed probe transmitters, the probe and the transmitter are constructed as a single unit, ensuring a specific, non-variable interface.
• Analog Communication: Many fixed probes, especially older models, operate on analog communication methods. This means that the transmitter receives raw signal outputs from the probe, which it then translates into humidity values.
• Internal Calibration Storage: Calibration and adjustment data for these probes is often stored within the transmitter, not in the probe itself.
• If the probe or sensor fails, the entire unit must be replaced. Likewise for calibration the ‘transmitter’ must be calibrated.

Interchangeable RH Probe Transmitters

• Modular Design: Probes are separate to the transmitter. This allows for a modular approach, selecting the right probe for your measurement application and the right transmitter for installation needs. Within Rotronic or Hygroclip range are all interchangeable probes.
• Digital Communication: A standout feature of Rotronic HygroClip probes is the utilization of digital communication from the probe to the transmitter. This ensures a faster, more reliable, and often more accurate data transfer.
• Internal Calibration Storage: Calibration and adjustment data in Rotronic HygroClip probes is stored within the probe. Digital communication between probe and transmitter allows the probe to maintain its accuracy and calibration even if switched between different transmitters. It also simplifies the recalibration process, as you're essentially calibrating the probe, not the transmitter. This dramatically decreases the downtime for calibration and maintenance. It should be noted that not all interchangeable probes from other manufacturers have digital communication and internal adjustment data.
• Unique Serial Numbers: Every interchangeable probe from Rotronic comes with a unique serial number, facilitating traceability and streamlined replacement processes.

Relative Humidity Transmitters

The landscape of relative humidity transmitters is vast, especially when deciding between fixed and interchangeable RH probe options.

Fixed RH Probe Transmitters

Benefits

• Lower Initial Cost: HF1 being the low-cost option for HVAC, the fixed probe transmitters are often less expensive upfront.
• Ease of Installation: These units come as a single piece, making installation straightforward and less time-consuming.
• Optimized Performance: Each probe is specifically designed and tuned to work seamlessly with its corresponding transmitter, offering reliable performance.
• Lower Calibration Cost/Time: With no need to swap out probes, calibration can be a quicker, more straightforward process.

Drawbacks

• Limited Flexibility: You're locked into the probe's performance characteristics; upgrading or changing the probe's capabilities isn't an option without replacing the entire unit.
• Maintenance Cost: If the probe fails, you may have to replace the whole transmitter, increasing long-term maintenance costs.
• Limited Application Range: These are generally designed for specific applications, making them less versatile.

Interchangeable RH Probe Transmitters

Benefits

Flexibility: One of the major advantages is the ability to swap probes based on the application requirements, such as high accuracy, high temperature, or high-pressure conditions.
• Future-Proofing: Newer or more advanced probes can be installed without changing the transmitter, providing a more future-proof solution.
• Lower Long-Term Maintenance & Calibration Costs: Faulty or outdated probes can be replaced individually, negating the need to replace the whole unit.
• Easier Calibration: Individual probes can be calibrated separately, allowing for continuous operation with a spare probe.

The Rotronic Transmitter HF5 and HF5-ex models can be adapted to a broader range of applications due to their compatibility with various types of HygroClip probes.

• Higher Initial Cost: The versatility and advanced features often require a higher initial investment (although it should be noted that calibration and maintenance costs usually far outweigh the initial investment over the lifetime of the instrument).
• Complexity: Handling different types of probes can require specialized knowledge and create complexity in maintenance.

Related Product Information
Probes and Sensors for Measurement of Relative Humidity
Transmitters for Humidity Measurement
Humidity and Temperature Calibration

Related Application Information
HVAC & Building Automation
Life Sciences & Pharmaceuticals
Food & Beverage Industry